1. Field of the Invention
The present invention relates to a liquid electrophotographic apparatus of the type in which a liquid toner is supplied over the photoconductive surface of a photoconductive material over which is formed an electrostatic latent image so that the latter is developed.
2. Description of the Related Art
So far there has been proposed liquid electrophotographic apparatus in which a liquid toner (comprising toner particles, an electric-charge adjusting agent, a dispersion agent and so on) is supplied over a photoconductive material such as an ELP master (comprising a lamination of a photoconductive layer comprising a photoconductive layer ZnO, a binder and a substrate which has been so processed as to be water-proof and electrically conductive), thereby developing an electrostatic latent image formed over the surface of the master.
In the electrophotographic apparatus of the type descibed above, an electrode pair consisting of two metal plates disposed in opposing relationship with each other and being spaced apart from each other by a desired distance (one of the metal plates, in opposing relationship with the photoconductive layer surface of the photoconductive material, is referred to as "a developing electrode", while the other metal plate, in opposing relationship with the back surface of the photoconductive material, is referred to as "a back plate", hereafter in this specification) is disposed, and a bias voltage applied between the electrode pair is short-circuited while the liquid toner is being supplied therebetween, and when the above-described master is passing therebetween, thereby developing the electrostatic latent image formed over the surface of the photoconductive layer.
When the liquid toner is supplied to the master passing between the electrode pair, the toner particles contained in the liquid toner are attracted by and adhere to the electrically charged particles forming an electrostatic image, whereby the latent image is made visible.
However, in the case of the conventional wet type photographic apparatus of the type described above, in response to the neutralization of the latent-image forming charged particles covering the surface of the master by the toner particles in the developing liquid, a charge is induced over the rear surface of the photoconductive layer of the photoconductive material, causing the charging of particles whose polarity is opposite to that of the charged particles over the photoconductive layer; that is, charged particles whose polarity is opposite to that of the toner particles, so that some toner particles adhere to the back electrode, thereby contaminating it.
A further problem resides in the fact that as the number of development operations increases, the quantity of the toner particles attached to the back electrode increases accordingly so that a nonelectrically conductive film is formed over the surface of the back electrode. Such a nonelectrically conductive film over the back electrode prevents the movement of the charged particles so that when the developing process is repeated many times, the film thus formed becomes electrically conductive because of the attachment thereto of the charged particles. As a result, the adhesion of the toner particles over the photoconductive surface is adversely affected, resulting in a decrease in the density of the image developed.
Furthermore, as the thickness of the layer of charged particles is increased, the layer breaks up into solid particles which in turn become attached to one or more portions of the master where no latent image is formed so that the toner particles on the surface of the master are contaminated.
In order to solve this problem, as best shown in FIG. 11. It has been proposed that the back surface of the substrate of the master be supported by a plurality of projections produced by roughening the surface of the back electrode. (Reference is made to Japanese Patent Application Laid-Open No. 61-11766.) In addition, a construction has also been proposed in which, as best shown in FIG. 12, an electrically conductive wire net is used as the back electrode. (Reference is made to Japanese Utility Model Application Publication No. 56-14525.) According to the above-described technique, the back electrode is brought into contact with the back surface of the master substrate so that the charged particles attached to the back electrode can be neutralized by those attached to the substrate, and so that the quantity of toner particles attached to the back electrode can be decreased.
However, the portions which are not brought into contact with the back surface of the substrate, but are instead spaced apart therefrom by a small distance or gap, so that the electric field produced between the back surface of the substrate and portions of the back electrode not in contact with the back surface of the substrate is increased, and the toner positively charged particles easily become attached to the back electrode (such a space left because of the noncontact relationship between the back electrode and the back surface of the substrate is referred as "a gap" hereinafter in this specification).
In the case of a back electrode whose surface is roughened, the discharge of the developing liquid between the back electrode and the master becomes difficult as a result of the many such "gaps" and a liquid film is formed on the surface thereof so that positive contact between the back electrode and the substrate cannot be achieved, resulting in the electrical attachment of positively charged particles to the back electrode.
In the case of the use of an electrically conductive wire net, it is difficult to maintain the wire net in the form of a flat surface, and thus, out of contact portions tend to occur frequently and the quality of the developed image cannot be stably and reliably ensured; that is, the quality is degraded. Even when a supporting member is provided, in order to maintain the flatness of the wire net, the liquid film is still formed in the manner described above, resulting in the same problem.
Japanese Patent Application Laid-Open No. 58-4165 discloses the disposing of the developing electrode in opposed relationship with the photoconductive layer from a foamed metal. According to this teaching, the primary object is to enable a lot of developing liquid to flow between the photoconductive layer and the developing electrode, by utilizing the fact that the degree of porosity of a foamed metal is generally high. In addition, the developing liquid is supplied from the side of the developing electrode so that the contact between the developing electrode and the photoconductive material is prevented. However, the back electrode is substantially similar in construction to the conventional back electrodes so that the attachment of the toner particles to the back electrode cannot be avoided.
The back electrode must be cleaned at a predetermined time interval, when a certain amount of developing liquid has passed over the photoconductive substrate, or, the material must be removed in the case of abnormal operation or the like. Furthermore, in the case of the conventional liquid electrophotographic apparatus, the developing stage must be completely disassembled. Because of these reasons, the maintenance of the electrophotographic apparatus is difficult, and the apparatus is not reliable and dependable. In addition, a further problem occurs when the photoconductive material is guided to the developing stage, namely, the back of the photoconductive material is contaminated by toner particles or the like attached to the guide surfaces.